1. Field of Invention
The U.S. government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DE-AC06-87RL10930 awarded by the United States Department of Energy.
This invention relates to the field of passively operated air flow control devices. More particularly to valves which maintain a constant air flow rate regardless of changes in the pressure at the inlet or outlet.
2. Description of Prior Art
Devices which maintain a constant air flow regardless of changes in the inlet or outlet pressures are referred to as constant air flow controllers or valves. Constant air flow control devices have a variety of uses in ventilation control systems. Currently there are several situations where maintaining a constant air flow despite varying pressures would be useful.
Currently, exhaust ventilation must be provided for any space in any facility in which gases, odors, noxious vapors, excessive heat or airborne particulates which are undesirable or pose a health hazard to the workers or the public are produced. Examples of such spaces may include: food, chemical or waste storage buildings; Waste processing buildings; vats, tanks, or rooms; enclosures for heat-producing equipment such as furnace, forges; laboratories; hospitals; classrooms and auditoriums; indoor swimming pools; wood and metalworking shops; welding booths; auto and equipment repair shops; kitchens; laundries; bathrooms and locker rooms.
In such spaces it is usually necessary to maintain a slight negative pressure relative to the surrounding spaces to remove the unwanted odor or hazardous substance. The pressure and air change rate in a given volume can be satisfied through the precise regulation of the air flow rate into or out of the ventilated space.
Conventional methods of controlling flow rates in exhaust ventilation systems involve the use of dampers in conjunction with a fan. These dampers are controlled either manually or automatically. However there are drawbacks to using dampers to control the ventilation in a given space. First, the task of controlling ventilation system flow rates with manual or weighted dampers can be difficult and continual. This is because the flow rates are easily affected by changes in system pressure, barometric pressure, wind velocities or system cleanliness (i.e., dirty filters). The second drawback is that air or motor operated valves or dampers which automatically control air flow rates are effective, but can be expensive to install.
Besides the use of flow valves in the exhausting of unwanted gases, a constant flow valve offers tremendous advantages in a forced-air heating and air conditioning system. Such heating and air conditioning systems are common in homes and office complexes.
In current practice, when forced-air heating and air conditioning systems are designed for offices and homes, air flow rates are calculated and the correct sizes heating and air conditioning unit are installed. When installation is completed, individual vents are adjusted from one room of office to another to equalize temperature.
But if, for example, the occupant of one office is too warm and closes a vent to cool the room, more warm air flows into other offices. This makes these offices too warm. Soon, adjustments are being made from one office to another. This results in the entire system being thrown out of balance and the occupants are uncomfortable. Sometimes a comfortable balance can be reached, but as soon as one more adjustment is made, or when the air conditioning is turned on, the whole cycle of adjustment repeats itself.
An economical air flow valve which could be designed to be placed in an individual room and that could be adjusted to maintain a desirable air flow without interfering with the air flow in other rooms would be an enormous improvement in the environment of many facilities.
The current art has attempted to provide several means to maintain a constant air flow rate, however, there are drawbacks.
One of the more advanced attempts to maintain a constant air flow involves the use of a sealed piston to exert a force necessary to regulate a flow area. Such a design is illustrated by U.S. Pat. No. 5,000,221 (Palmer). However, there are several drawbacks to the Palmer design. In order for Palmer to maintain a constant mass flow at the outlet port, a seal must be provided between the piston and its mounting structure to prevent internal venting. The sealing methods described include the use of a diaphragm, a magnetic sealing fluid, or a fine machined tolerance between the piston and piston mounting structure. This seal makes the design unnecessarily complex and inhibits accuracy and reliability (due to the risk of failing or worn sealing mechanisms). The fine machined tolerance, besides being expensive can result in increased sensitivity to dirt or deposits. The dirt or deposits can cause great inaccuracies by preventing the piston from moving freely. However, relaxing this tolerance could allow too much leakage within the device and result in failure to maintain a constant mass flow through the device. Additionally, the Palmer device will not operate when the pressure at the outlet port is less than 2" wg. Finally, the maximum flow capacity of the Palmer device is 200 scfm.